1. Field of the Invention
This invention relates to a method for producing a polymer by coordination polymerization.
2. Description of the Related Art
When the catalyst combining titanium tetrachloride and triethylaluminum (Ziegler catalyst) is used to polymerize ethylene, the polymerization proceeds at a low pressure while an ethylene monomer is being coordinated to the metal of the catalyst and produces polyethylene having very small degree of branching. The mechanism of such polymerization is referred to as xe2x80x9ccoordination polymerizationxe2x80x9d and it is a very effective method for producing polyolefins with high stereoregularity.
In the coordination polymerization of olefin monomers, widely used are, aside from Ziegler catalyst, Natta catalyst combining titanium trichloride and triethylaluminum, the so-called Kaminsky catalysts that are represented by a combination of bis(cyclopentadienyl)titanium dichloride (Cp2TiCl2) and methylaluminoxane (the reaction product of methylaluminum and water), and the like. They can produce a variety of polyolefins having remarkably high stereoregularity and excellent physical properties as compared with polyolefins that are polymerized by radical polymerization methods.
It is also known that a polymerization of ethylene proceeds rapidly at 25xc2x0 C., and 1 atm pressure to form a polyethylene with high stereoregularity when tris(pentafluorophenyl)boron (B(C6F5)3) is used as a cocatalyst in combination with zirconocene catalyst such as bis(cyclopentadienyl)dimethylzirconium (Cp2ZrMe2) (Xinmin Yang, et al., J. Am. Chem. Soc. 1991, 113, 3623-3625).
Recently, there have been developed coordination polymerization catalysts (the so-called Brookhart catalysts) that are represented by xcex1-diimine nickel complexes; thus, they have made it possible to copolymerize olefin monomers with various unsaturated monomers.
However, a problem with the coordination polymerization catalysts described above is that it is difficult for polar monomers, especially unsaturated monomers capable of releasing a proton, to undergo coordination polymerization. Thus, when an unsaturated monomer, e.g., acrylic acid, having a functional group capable of releasing a proton and an olefin monomer, e.g., ethylene, are to copolymerize in the presence of an above described catalyst, the reaction does not take place at all, or at least, the yield of the product will be very low, because the unsaturated monomer having a functional group capable of releasing a proton lowers the reactivity of the coordination polymerization catalyst.
On the other hand, radical polymerizations are not easily influenced by the unsaturated monomers having functional groups capable of releasing a proton; therefore, use of the radical polymerization will enable the above-described monomers to copolymerize. However, the problem of low crystallinity of the resulting copolymers arises, because of their insufficient stereoregularity. In addition, since radical polymerization requires high temperature and high pressure reaction conditions, they are not favorable economically.
In an attempt to overcome these disadvantages, U.S. Pat. No. 5,153,282 discloses a method to convert a polar mononer into a masked monomer utilizing a masking agent comprising a non-halogenated metallic compound and to use the masked monomer for polymerization.
This invention has been made in view of these technical problems and is directed at providing a method for the production of a polymer which allows even a monomer having a functional group releasing a proton, that lowers the reactivity of a coordination polymerization catalyst, to undergo coordination polymerization.
The present inventors have repeated thorough investigations in order to attain the objective described above. As a result, it has been discovered that after a monomer having a functional group releasing a proton, that lowers the reactivity of a coordination polymerization catalyst, has been masked with a masking agent comprising
(i) a halogenated organometallic compound having a specific chemical structure or
(ii) a organoaluminum compound having a specific chemical structure
to form a masked monomer, the use of this masked monomer as a polymerization monomer will permit the production of a polymer through coordination polymerization. This invention has thus been accomplished.
Specifically, the first embodiment of the method for producing a polymer by coordination polymerization according to this invention is characterized in that it comprises the steps of:
(a) reacting an unsaturated polar monomer having a functional group releasing a proton or a salt of the unsaturated polar monomer, with a masking agent comprising an organometallic compound of the general formula (I) capable of masking the functional group to form a corresponding masked monomer;
Rnxe2x80x94Mxe2x80x94Xmxe2x80x83xe2x80x83(I)
wherein M is a metal selected from the group consisting of Li, Mg, Ca, Sc, Ti, V, Cr, Fe, Ni, Cu, Zn, Co, Al, Si, Y, Zr, Rh, Pd, In, Sn, Sm, and Hf; R is a ligand represented by a compound or an ionic moiety thereof selected from the group consisting of an unsaturated cyclic hydrocarbon, an unsaturated cyclic hydrocarbon having hydroxyl group, an unsaturated heterocyclic hydrocarbon and an unsaturated heterocyclic hydrocarbon having hydroxyl group; X represents a halogen atom; n is an integer of 1 to 5; m is an integer of 1 to 3; xe2x80x9cn+mxe2x80x9d is an integer of 2 to 6;
further wherein R may be the same or different when n is an integer of 2 to 5; and
(b) polymerizing the masked monomer either alone or in combination with another monomer in the presence of the coordination polymerization catalyst to obtain the polymer.
The second embodiment of the method for producing a polymer by coordination polymerization according to this invention is characterized in that it comprises the steps of:
(a) reacting an unsaturated polar monomer having a functional group releasing a proton or a salt of the unsaturated polar monomer, with a masking agent comprising an organometallic compound of the general formula (II) capable of masking the functional group to form a corresponding masked monomer;
R1xe2x80x94Al"Brketopenst"Oxe2x80x94R2]2xe2x80x83xe2x80x83(II)
wherein R1 is a monovalent organic group having 1-6 carbons and R2 is a group selected from an unsaturated heterocyclic hydrocarbyl group and a phenyl group substituted with one or more organic groups having 1 or more carbons wherein the substitution at least takes place at the 2- or the 6-position of phenyl ring; and
(b) polymerizing the masked monomer either alone or in combination with another monomer in the presence of the coordination polymerization catalyst to obtain the polymer.
The first and second embodiments of the method for producing a polymer of the invention may further comprise the step of removing the masking agent from the polymer after the step of obtaining the polymer by the use of the coordination polymerization catalyst.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.